The internal mammary artery (IMA) is a superior conduit for coronary artery revascularization and many factors have been suggested for explanation of this superiority. IMA and saphenous vein grafts have been systematically analysed with scanning electron microscopy (SEM) in a series of 11 patients undergoing coronary artery revascularization. At the time of implantation endothelial damage is almost absent in internal-mammary-artery (IMA) grafts; small areas of exposed subendothelial matrix may be present but are essentially non-thrombogenic as reflected by the lack of clots in these areas. In contrast the endothelium of harvested human saphenous veins (SV) shows large thrombogenic defects with exposed collagenous fibrils. The extent and deepness of the defects deteriorated in the period between removal of the vein and its attachment to the aorta. We conclude that long-term superiority of IMA grafts may also be due to the lack of primary intimal defects.